In the past more than 20 years burners with short but effective premixing zones (so-called EV burners: environmental friendly V-shaped burners) have been implemented in several gas turbines of the applicant, with very low NOx levels. In addition to this, three variants of premix technologies have been successfully developed and deployed into those gas turbine engines: the sequential EV burners—a technology that allows premixing of natural gas and oil into a hot exhaust stream to reheat the exhaust gases of a first high pressure turbine; the MBtu EV burners that are used to burn syngas in a premix flame with low NOx emissions; and the advanced EV burners (AEV) that are capable to prevaporize and premix liquid fuel prior to combustion and burn it with very low NOx emissions without water injection.
Document EP 0 851 172 A2 discloses an exemplary EV burner of the double-cone type, for operating a combustion chamber with a liquid and/or gaseous fuel, whereby the combustion air required for this purpose is directed through tangential air-inlet ducts into an interior space of the burner. This directing of the flow results in a swirl flow in the interior space, which swirl flow induces a backflow zone at the outlet of the burner. In order to stabilize the flame front forming there, at least one zone is provided at each sectional body forming the burner, within which zone inlet openings are provided for the injection of supplementary air into the swirl flow. Due to this injection, a film forms at the inner wall of the sectional bodies, which film prevents the flame from being able to flashback along the inner wall of the sectional bodies into the interior space of the burner.
Document EP 2 423 597 A2 shows another exemplary EV burner in the form of a double-cone burner, which has two partial cone shells which are arranged nested one inside the other, forming air inlet ducts between them, through which combustion air from the outside flows into a conical inner space of the premix burner. Linear rows of holes of injection openings, which extend transversely to the flow direction of the combustion air, are arranged on the outer walls of the air inlet ducts and through which a gaseous fuel is injected into the combustion air which flows past transversely to them.
Document DE 195 45 310 A1 disclose a further pre-mixing burner consisting of a hollow cone with an outer and inner cone casing. At least two inlet ducts run at a tangent to the inner cone casing and are positioned along a straight cone casing line. The part cone axes of the part shells formed lie on the same cone axis. The pre-mixing burner is divided into at least two, for example four, parts containing the inlet ducts so as to swirl the combustion air. A fuel nozzle is positioned at the cone tip for injecting liquid fuel.
The main design parameters for the current EV and AEV burners with a straight cone are the exit diameter, the slot width and the cone angle. These parameters are chosen such, that for a given throughflow capacity and pressure loss the vortex breakdown occurs near to the burner exit. The constraint is that the swirl strength is increasing linearly along the axis until it reaches the critical swirl strength for vortex breakdown near to the burner axis. This means that with these parameters the burner length and burner envelope is preset.